Method of making photochromic glasses suitable for simultaneous heat treatment and shaping

ABSTRACT

The instant invention is related to the manufacture of photochromic glasses having base compositions within a very narrow interval of the alkali metal boroaluminosilicate system wherein silver chloride and/or silver bromide crystals impart photochromic properties. The glasses are notable for their rapid fading characteristics and relatively low temperature dependence of darkening. The compositions are especially suitable for a production process which contemplates simultaneously shaping articles from glass sheet and developing photochromic properties therein.

This is a continuation of application Ser. No. 947,502, filed Oct. 2,1978, now abandoned, which is a division of application Ser. No.895,646, filed Apr. 12, 1978, now U.S. Pat. No. 4,130,437.

BACKGROUND OF THE INVENTION

The field of photochromic glasses is founded in U.S. Pat. No. 3,208,860which discloses the production of silicate-based glasses that exhibitdarkening when exposed to actinic radiation, customarily ultravioletradiation, and which return to their original color when removed fromthe source of actinic radiation. Such reversible optical properties areachieved via the incorporation of effective amounts of silver and atleast one halide of the group chloride, bromide, and iodide into theglass composition which combine to form silver halide crystallites inthe glass. The crystallites are so small as to be invisible to theunaided eye, yet are darkenable under the action of actinic radiation toreduce the optical transmittance of the glass. When the actinicradiation is extinguished, the crystallites fade to their originalstate, thereby restoring the optical transmittance to its initial level.This cycle of darkening and fading can be repeated indefinitely withoutfatigue in photochromic glasses.

By far the most prevalent use for photochromic glasses has been in thefabrication of ophthalmic lenses. One example of that application isprovided in U.S. Pat. No. 3,197,296 which describes a family ofrefractive index-corrected silicate glasses containing silver halidecrystals to provide the desired photochromic behavior. Those glassesdemonstrated, in conventional 2 mm thickness, photochromic propertiessufficiently developed for prescription ophthalmic applications alongwith the necessary refractive index to be compatible with conventionallens grinding practices.

The manufacture of ophthalmic lenses commonly involves the pressing ofglass lens blanks of optical quality from a melt followed by thegrinding and polishing of the blanks to predetermined prescriptions. Itis believed self-evident that the production of non-prescriptionphotochromic glass lenses, for example, sunglass lenses, in largequantities by processes demanding grinding and polishing is not onlyexpensive and time consuming, but is also wasteful of material.Consequently, less costly means for producing photochromic glass sheetfor lenses or other applications would be highly desirable. Assumingthat the sheet could be produced in optical quality, the sheet could beinexpensively thermally sagged to the curvatures required for lenses,windshields, and other sheet glass configurations.

The commercial sheet glass forming processes practiced today contemplatemaintaining substantial volumes of molten glass at temperatures whereinthe glass has the necessary viscosity for sheet forming, viz., at aviscosity between about 10⁴ -10⁶ poises. By the very nature of thedrawing process, those volumes of glass are in prolonged contact withrefractory metals or ceramics which serve as the means for forming drawnsheet. Thus, the sheet drawing processes impose severe constraints uponglass composition because of the formidable liquidus and glass stabilityproblems associated with the handling and processing of glass atrelatively low temperatures and high viscosities.

In addition to good formability properties, suitable glass sheet forophthalmic purposes will exhibit high optical quality, good chemicaldurability, high strength, and good photochromic darkening even in sheetof moderate thickness. Where the sheet is scheduled for use aslightweight sunglass lenses, the glass must also be chemicallystrengthened such as to meet the Food and Drug Administration (FDA)requirements for eyeglass lens safety. Federal safety requirementscannot be routinely met in lightweight glass of moderate thicknesses(1.3-1.7 millimeters) in the absence of chemical strengthening, or byutilizing an air tempering procedure. U.S. Pat. No. 4,018,965 describesa group of glass compositions which demonstrates the propertiesnecessary for photochromic sheet glass applications.

From the considerable experience gained through the years in themanufacture of photochromic glasses suitable for ophthalmicapplications, the following several criteria have been formulatedtherefor as goals to achieve in the production of sunglasses; thesecriteria being in addition to the necessary melting, forming, andchemical strengthening capabilities, as well as the physicalcharacteristics conventionally demanded in non-photochromic ophthalmicware.

First, a glass which in 1.5 mm thickness at room temperatures (25°-30°C.) will exhibit an optical transmittance in the range of 60-90% beforeexposure to actinic radiation but which, when irradiated with actinicradiation, e.g., bright outdoor sunlight, will darken to a transmittanceof less than 30%.

Second, a glass which in 1.5 mm thickness at 25°-30° C. will fade veryrapidly when removed from the incident actinic radiation; i.e., theglass within five minutes will fade to a transmittance of about twotimes its darkened transmittance and within an hour will fade to atransmittance of at least 80% of its original transmittance.

One circumstance which must be kept in mind when conducting researchinvolving photochromic glass is the fact that the dynamics ofphotochromic behavior exhibited by glasses are directly related to theintensity of the actinic radiation impinging thereon and the temperatureof the glass while being irradiated. Accordingly, where other parametersare held constant, a photochromic glass will customarily darken to alower transmittance when exposed to actinic radiation while at a lowertemperature. Moreover, the intensity of solar radiation can obviouslyvary greatly depending upon the season of the year, the location of theexposure (angle of declination of the sun), cloud cover, snow cover, airmass value, etc.

With respect to temperature dependence, i.e., the degree of darkeningdemonstrated by a photochromic glass over a range of ambienttemperatures, some photochromic glasses in 1.5 mm thickness may darkento a transmittance of less than 5% when subjected to solar radiation ata temperature of -18° C. (0° F.). Such glasses would not comply with thespecifications of the American National Standards Institute (ANSI) whichspecify lenses for general use as fixed tint sunglasses to exhibit anoptical transmittance of at least 5%.

Consequently, a third criterion proposed for photochromic glasses whichare to be used for ophthalmic applications is that in 1.5 mm thicknessthe glass will not darken to a transmittance of less than 5% at -18° C.

The converse of the above-stated rule regarding temperature dependencealso holds true; viz., where other parameters are maintained constant, aphotochromic glass will darken to a lesser degree, i.e., the finaldarkened optical transmittance will be higher, when the glass is at ahigher temperature when exposed to actinic radiation. To have practicalutility as a sunglass, it has been deemed that a photochromic glassshould darken to an optical transmittance of less than 50% when exposedto outdoor sunlight at temperatures encountered during summer.

Accordingly, a fourth criterion which has been proposed is that aphotochromic glass in 1.5 mm thickness will darken to a transmittanceless than 50% when exposed to actinic radiation at 40° C. (104° F.).

Finally, to simplify manufacturing techniques, while concomitantlymaintaining the optical properties of the pristine glass surface, theideal glass compositions would permit the desired photochromicproperties to be developed concurrently with the required lens curvatureduring a thermal sagging operation. U.S. application Ser. No. 773,958,filed Mar. 3, 1977 in the names of Bourg, Hazart, and Jouret now U.S.Pat. No. 4,088,470, discloses such a technique for simultaneously heattreating and sagging sheet of photochromic glass into lens blanks of adesired curvature.

It is believed apparent from the prior art that the photochromicproperties exhibited by a particular glass are dependent upon bothcomposition and the heat treatment to which it is subjected. Thecurvature secured in a thermal sag cycle is also a function of suchparameters as glass composition and incident thermal cycle resultingthrough the combined effects of surface energy, density, and viscosity,this latter factor being strongly dependent upon temperature. A mostfortuitous circumstance would exist where the desired photochromicbehavior could be achieved through the same heat treatment as thatgiving rise to the necessary lens curvature.

Therefore, a fifth criterion proposed is a glass capable of beingconcurrently heat treated and sagged to simultaneously yield the desiredlens curvature and photochromic properties.

U.S. application Ser. No. 887,677, filed Mar. 17, 1978 by G. B. Hares,D. L. Morse, D. W. Smith, and T. P. Seward, III, now abandoned andrefiled Feb. 28, 1979 as U.S. application Ser. No. 14,981, now U.S. Pat.No. 4,190,451 discloses a silver halide-containing, silicatephotochromic glasses exhibiting quite rapid fading characteristics andrelatively low temperature dependence of darkening. Several of thecompositions recited in that application are operable for sheet drawingprocesses but are not suitable for a simultaneous heat treating-saggingprocedure, such as has been described above. The inapplicability ofthose glasses for such a process resides in the fact that the times andtemperatures demanded to sag the glass sheet are such as to cause theglass to sag into contact with formers which produce the necessary lenscurvature, this contact causing the destruction of the good opticalproperties of the pristine surface. Yet, without such formers, thoseglasses would sag to a much higher curvature than desired. Thus, thelens blanks fabricated from those glass compositions via a heattreating-sagging technique would require grinding and polishing toprovide the required optical quality surface.

OBJECTIVE OF THE INVENTION

The principal objective of the instant invention is the manufacture oftransparent photochromic glass which, in sheet form, will be suitablefor the fabrication of sunglasses through a heat treating-saggingprocess and which, in 1.3-1.7 mm thicknesses, manifests the followingphotochromic and physical properties:

(a) at about 25°-30° C., the glasses will darken to a luminoustransmittance below 30% in the presence of actinic radiation, e.g.,bright outdoor sunshine; the glasses will fade to a luminoustransmittance at least 1.75 and, preferably, two times the darkenedtransmittance after five minutes' removal from the actinic radiation;and the glasses will fade to a luminous transmittance in excess of 80%of their original undarkened transmittance in no more than one hourafter being removed from the actinic radiation;

(b) at about 40° C., the glasses will darken to a luminous transmittancebelow 50% in the presence of actinic radiation, e.g., bright outdoorsunshine, and will fade to a luminous transmittance in excess of 80% oftheir original undarkened transmittance in no more than one hour afterbeing removed from the actinic radiation;

(c) in the undarkened state, the glasses will exhibit a luminoustransmittance (clear luminous transmittance) of at least 60%,conveniently obtained by incorporating a fixed tint in the composition,but more typically within the range of 85-92%;

(d) at about -18° C., the glasses will not darken to a luminoustransmittance below 5% in the presence of actinic radiation, e.g.,bright sunlight;

(e) the glasses are capable of being strengthened via either thermaltempering or chemical strengthening while maintaining the desiredphotochromic properties; and

(f) the glasses in sheet form have the capability of beingsimultaneously heat treated and sagged to produce lens blanks of theproper curvature with the desired photochromic properties.

SUMMARY OF THE INVENTION

In the most general terms, the glass compositions operable in thepresent invention consist essentially, in weight percent on the oxidebasis as calculated from the batch, of about 54-66% SiO₂, 7-15% Al₂ O₃,10-25% B₂ O₃, 0.5-4.0% Li₂ O, 3.5-15% Na₂ O, 0-10% K₂ O, 6-16% total ofLi₂ O+Na₂ O+K₂ O, 0-1.25% PbO, 0.10-0.3% Ag, 0.2-1.0% Cl, 0-0.3% Br,0.002-0.02% CuO, and 0-2.5% F. The glass may optionally additionallycontain colorant oxides selected in the indicated proportions from thegroup consisting of 0-1% total of transition metal oxide colorants and0-5% total of rare earth oxide colorants.

Glasses produced from the above-described compositions exhibitviscosities of at least about 10⁴ poises at the liquidus temperature,thereby providing a liquidus-viscosity relationship permitting formingvia direct sheet drawing from the melt. The glasses also demonstratelong term stability against devitrification in contact with platinum attemperatures corresponding to glass viscosities in the range of 10⁴ -10⁶poises, and, hence, can be drawn from a melt at those viscositiesutilizing platinum or platinum-clad drawbars, downdraw pipes, or othersheet forming means to yield glass sheet of optical quality. As definedherein, long term stability against devitrification comprehends goodresistance to surface crystal growth in contact with platinum attemperatures corresponding to glass viscosities in the 10⁴ -10⁶ poiserange. The growth of a crystalline layer not exceeding 10 microns inthickness at the glass-platinum interface over a contact period of 30days at those viscosities is considered good resistance to crystalgrowth.

The inventive glasses also display excellent chemical durability, bywhich is meant that the glasses manifest no visible surface filmformation or iridescence following a 10-minute exposure at 25° C. to 10%by weight aqueous HCl.

Glasses within the above-recited composition area are capable of beingchemically strengthened to modulus of rupture values in excess of about45,000 psi with a depth of ion-exchanged layer of at least 0.0035", asdetermined by conventional stress layer examination techniquesemploying, for example, a polarizing microscope equipped with a Babinetcompensator. Such strength and compression layer characteristics can besecured through conventional sodium-for-lithium salt bath ion exchangeprocesses at normal ion exchange temperatures (300°-450° C.), thesurface compression being generated by the replacement of Li⁺ ions inthe glass surface with the larger Na⁺ ions of the molten salt. Suchphysical properties permit glass sheet of 1.3-1.7 mm thickness toreadily pass the Food and Drug Administration impact test for ophthalmiclenses (the drop of a 5/8" steel ball from a height of 50").

Finally, glasses within the inventive composition region exhibit anexcellent combination of photochromic properties following heattreatment in accordance with conventional practice. These propertiesinclude, in glass sheet not exceeding about 1.7 mm thickness, a darkenedluminous transmittance of less than 30% at 25°-30° C., a luminoustransmittance after five minutes' removal from actinic radiation of atleast 1.75 and, preferably, two times that of the darkened state, and aluminous transmittance after one hour's removal from actinic radiationof at least 80% of their original undarkened transmittance. Uponexposure to actinic radiation at -18° C., the luminous transmittance ofthe glasses will not fall below 5%. At 40° C., exposure to actinicradiation will darken the glasses to below 50% transmittance and theglasses will fade to a luminous transmittance in excess of 80% of theiroriginal undarkened transmittance.

For the purposes of the present description, the luminous transmittanceof a glass is defined as the value Y delineated in terms of the 1931C.I.E. trichromatic colorimetric system utilizing the light sourceIlluminant C. This colorimetric system and light source are described byA. C. Hardy in the Handbook of Colorimetry, Technology Press, M.I.T.,Cambridge, Massachusetts (1936). Also, as employed in this disclosure,the clear or undarkened state is obtained via an overnight (at least 8hours) fading of the glass in the absence of light. A slightly clearerglass (2-3 percentage transmittance units higher) can be secured bysubmerging the glass in boiling water for five minutes.

Glass designed for sunglass lens applications will preferably exhibit aclear luminous transmittance of at least about 60%, this value beingreadily obtainable in the inventive glasses in combination with theother desired photochromic properties. Darker glasses having clearluminous transmittances of less than 60%, however, can be achievedwithin the inventive glass composition interval where maximum or nearmaximum concentrations of the cited colorants are included.

The method of the instant invention comprises an improved process forthe production of drawn photochromic glass sheet wherein a glass-formingbatch is melted, the melt adjusted in temperature to provide a viscosityof 10⁴ -10⁶ poises, and then drawn past refractory forming means withinthat range of viscosities to yield potentially photochromic glass sheet.As used herein, potentially photochromic glass sheet is defined as glasssheet including silver halides and sensitizing agents or activators suchas copper oxide which can be rendered photochromic via an appropriateheat treatment after the forming step. The glass sheet can be formedutilizing conventional updraw or downdraw processes.

Observance of the inventive compositional and process parameters,coupled with supplemental heat treatments and strengthening proceduresinvolving conventional time-temperature schedules, permits theproduction of chemically strengthened photochromic drawn sheet glassarticles which can be especially suitable for the fabrication of thin,lightweight photochromic ophthalmic or sunglass lenses. Mostimportantly, the above-described compositional and process parametersenable the potentially photochromic sheet to be rendered photochromicduring a heat treatment schedule designed to produce sagged lenses forophthalmic or sunglass applications. Thus, the imparting of photochromicbehavior to the glass and the sagging thereof to the proper curvatureare accomplished in the same heat treatment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Inasmuch as the chemical, photochromic, and physical properties, alongwith sagged lens curvatures (when required), are complex functions ofthe several constituents of the glass composition, strict adherence tothe compositional limitations of the inventive glasses is vital toachieving the desired combination of properties.

As was observed in U.S. Pat. No. 4,018,965, supra, the presence of Li₂ Ois demanded in the glass composition to secure the capability of beingchemically strengthened. Hence, where less than about 0.5% by weight Li₂O is present in the composition, modulus of rupture values in excess ofabout 45,000 psi and depth of compression layers of 0.0035" cannot beconsistently obtained. On the other hand, Li₂ O contents in excess of 4%by weight give rise to decreased glass stability against platinum metalwhen the molten glass has a viscosity within the 10⁴ -10⁶ poiseinterval, and hazards the development of haze in the glass. The desiredmechanical strength and depth of compression layer cannot be attained inthe absence or near-absence of Li₂ O employing, for example, a K⁺-for-Na⁺ ion exchange treatment to strengthen the glass.

Control must be maintained over the levels of the other alkali metaloxides because of their effect upon both photochromic and chemicalstrengthening characteristics. For example, where less than the statedconcentrations of Na₂ O and K₂ O are present, photochromic darkenabilityand the capacity for chemical strengthening are impaired. Quantities ofalkali metal oxide greater than the total specified act to reduce thefading rate of the glass and K₂ O in excess of the stated limit seems toreduce the chemical strengthening potential of the glass.

The presence of Al₂ O₃ and B₂ O₃ in the composition appears to counterthe adverse effect upon fade rate exercised by the alkali metal oxides.Hence, glasses containing less than the recited amounts of thosecomponents will generally demonstrate inferior photochromic behavior.The inclusion of more than about 25% by weight B₂ O₃, however, tends todecrease the chemical durability of the glass. Where more than about 15%by weight Al₂ O₃ is employed, the glass stability againstdevitrification is substantially degraded, the excess Al₂ O₃ being proneto combine with the Li₂ O of the composition to produce spodumene solidsolution crystals.

The presence of lead oxide in the specified range can be of greatsignificance in providing the desired combination of photochromicproperties in the glass, particularly with regard to the amount ofdarkening and the fade rate, as will be discussed in more detail infra.

The addition of minor amounts of compatible constituents to the glasscomposition is permissible but is generally avoided because of thepossibility of adversely affecting the desired combination ofphotochromic and physical characteristics. Accordingly, whereas alkalineearth and other multivalent metal oxides may be included, no substantiveproperty advantages have been perceived in so doing and, frequently,such additions tend to increase the liquidus temperature and decreasethe long term stability of the glass. Minor amounts of the alkali metaloxides Rb₂ O and Cs₂ O may be added, but such appear to impair thechemical strengthening potential of the glass.

TiO₂ and ZrO₂ will preferably be entirely absent due to their knownfunction as a nucleating agent for crystal growth. As little as 0.8%ZrO₂ can promote zircon crystallization at temperatures in the glassforming range.

Additions of SnO₂, Sb₂ O₃, and/or As₂ O₃ may be useful in modifying thecharacteristics of the glass melt, particularly with regard to theoxidation state thereof.

As has been disclosed above, lead oxide can play an important role incontrolling photochromic properties. Improved darkening of the glass issecured when PbO is present in an amount of at least 0.15%. The fastestfading glasses contain PbO in levels less than about 0.7% by weight.

A very significant facet of the instant invention is the discovery thatcopper at concentrations of 0.002-0.020% by weight CuO can play ameaningful part in achieving low temperature dependence of darkeningwithout degradation in fade rate. Consequently, when the composition ofthe base glass is changed to modify the physical properties thereof,and, in so doing, the content of PbO or alkali metal oxide is increased,an increased amount of copper will be required to achieve the optimumcombination of darkening, fading, and low temperature dependence.

Where a simultaneous heat treat-sag processing step constitutes anelement in the line of production, silver and bromide, as analyzed inthe glass, should fall within the indicated ranges of 0.12-0.18% and0.060-0.13%, respectively. Smaller amounts of Ag and Br do not providesufficient nucleation and, as a result, the glasses tend to be hazy anddarken poorly. With greater quantities of Ag and Br, nucleation isexcessive and the glasses do not darken well when heat treated for onlythe short periods of time required to give good sagged lens curvature.

Good darkening character has been found consistent with high chlorideconcentrations. Thus, Cl levels greater than 0.2% and, preferably, inexcess of 0.3% by weight are required. Nevertheless, because highchloride contents appear prone to increase the temperature dependence ofthe glass somewhat, discretion dictates that the chloride concentrationsbe kept at such low levels as is practically consistent with gooddarkenability.

The inventive glass compositions can be compounded from conventionalglass bath constituents in proportions that will yield the desired oxidecomponents in the proper amounts at temperatures utilized for meltingthe glass. The melting may be undertaken in accordance with conventionaloptical glass melting practice in crucibles, pots, tanks, or othermelting units at temperatures within the 1200°-1550° C. interval.

The molten glass may be formed utilizing any of the techniques known tothe glassmaking art such as blowing, casting, pressing, rolling, andspinning. Moreover, the glass is sufficiently stable that it may also beformed into sheet by direct drawing from the melt, at least whereplatinum or other refractory metal-lined drawbars, downdraw troughs, orother forming means are utilized.

The glass sheet or other articles may then be heat treated in accordancewith thermal schedules conventional for photochromic glasses in order todevelop the desired photochromic behavior therein. Thus, operable heattreatments contemplate exposure of the glass sheet to temperatureswithin the range of 580°-750° C. for times ranging from a few seconds toa few hours. To insure the required optical surface quality, the glasswill be supported in a manner calculated to preclude surface marking as,for example, via edge support means.

Where the glass will be sagged to the desired lens curvature and thephotochromic properties developed simultaneously during the same heattreatment, temperatures between about 610°-640° C. for periods of timebetween about 6-15 minutes or about 640°-660° C. for about 5-12 minuteshave been found suitable. Lens curvatures of about 4-6 diopters in 60-80mm diameter lenses have been developed.

Finally, after the photochromic properties have been generated, theglass articles can be subjected to conventional chemical strengtheningtreatments; for example, immersion in a bath of molten NaNO₃ or a bathof molten NaNO₃ +KNO₃ containing at least 30% by weight of NaNO₃. Thedesired strength and depth of compression layer can be attained wherethe immersion is conducted for about 4-24 hours in baths at temperaturesbetween about 300°-450° C.

The most optimum combination of photochromic and physical properties,wherein lenses are simultaneously sagged to the required curvatures andphotochromic properties are developed therein, and those lenses aresubsequently chemically strengthened without substantial impairment ofthe photochromic properties, is produced within a preferred group ofglasses having compositions consisting essentially, in weight percent onthe oxide basis as calculated from the batch, of about 57.1-65.3% SiO₂,9.6-13.9% Al₂ O₃, 12.0-22.0% B₂ O₃, 1.0-3.5% Li₂ O, 3.7-12.0% Na₂ O,0-5.8% K₂ O, 6-15% total of Li₂ O+Na₂ O+K₂ O, a molar ratio of Li₂ O:Na₂O+K₂ O not exceeding about 2:3, 0-1.25% PbO, 0.12-0.24% Ag, 0.2-1.0% Cl,0.06-0.25% Br, 0-2.5% F, 0.002-0.020% CuO, 0-1.0% total of transitionmetal oxides selected in the indicated proportions from the groupconsisting of 0-0.5% CoO, 0-1.0% NiO, and 0-1.0% Cr₂ O₃, and 0-5.0%total of rare earth metal oxides selected from the group consisting ofEr₂ O₃, Pr₂ O₃, Ho₂ O₃, and Nd₂ O₃.

Specific examples of preferred glass compositions falling within theabove ranges are reported in Table I below. The individual componentsare expressed in parts by weight on the oxide basis as calculated fromthe batch, except that the halides and silver are tabulated on anelemental basis in accordance with customary glass analysis practice.Analyzed values are also recorded for Ag, Br, and Cl since it is theretained concentrations of those ingredients which are critical to theinvention. The values to the left of the slash mark represent batchcontent and those to the right of the slash mark analyzed levels. Wetchemical and X-ray emission techniques were employed in those analyses.Inasmuch as the sum of the several ingredients approximates 100, for allpractical purposes the values reported can be deemed to represent weightpercent.

Each of the glasses within the above range of preferred compositions,including the specific examples of Table I, has a viscosity at theliquidus of at least 10⁵ poises, as well as excellent chemicaldurability as characterized by essential inertness in theabove-described acidic solutions. The glass also demonstrates long termstability against devitrification in that it manifests good resistanceto crystallization when in contact with platinum at viscosities withinthe 10⁴ -10⁶ poise range. Furthermore, all of the preferred glasses arecapable of being chemically strengthened to modulus of rupture values ofat least 45,000 psi with a depth of compression layer of at least0.0035", utilizing conventional ion exchange strengthening techniques.

Table I also includes photochromic property data measured on individualsamples at 27° C., 40° C., and -18° C., wherein Y_(o) indicates theclear luminous transmittance of the glass, Y₁₀ and Y₂₀ represent thedarkened luminous transmittances of the glass after 10 and 20 minutes'exposure, respectively, to actinic radiation, and Y_(F5) reports theluminous transmittance of the darkened glass after a five-minute removalfrom the actinic radiation.

In the past an ultraviolet lamp has been used as a convenient source ofactinic radiation to test the photochromic characteristics of glasssamples, since it was recognized that photochromic glasses wereprimarily activated by radiations in the ultraviolet and low visibleportions of the spectrum. It has been found, however, that frequentlythe correlation between the data obtained with the ultraviolet lamp andthe values measured from solar radiation outdoors was poor.Consequently, in order to secure better correlation with outdoor solarexposure, a "solar simulator" was devised for the determination of theluminous transmittance Y in Table I.

The solar simulator apparatus, as described in U.S. application Ser. No.839,496, filed Oct. 5, 1977 in the name of Chodak, now U.S. Pat. No.4,125,775, is based around a 150 watt xenon arc source fitted with afilter to modify the spectral output thereof so as to closely approachthe solar spectrum, particularly in the ultraviolet, blue, and redportions. The infrared region of the spectrum is attenuated with a layerof water of sufficient thickness to provide equal irradiance to that ofthe sun, but with no special regard for the spectral distribution withinthat region.

The intensity of the arc source was adjusted such that the amount ofdarkening resulting from exposure to the light source was essentiallyidentical to that of a number of commercially available photochromicglasses, including PHOTOGRAY® lenses, darkened outdoors at noon during acloudless early summer day in Corning, New York (air mass value of about1.06). Numerous experimental photochromic glasses of widely-variantcompositions were also subjected to the solar simulator and to outdoorsunlight. Good overall agreement was observed in comparisons between thetwo types of measurements.

In order to continuously monitor the darkened transmittance of thespecimens, each sample was interrogated with a chopped beam of lightfrom a tungsten-halogen lamp detected by a PIN silicon photodiode whoseoutput was demodulated via a lock-in amplifier. A composite color filterwas placed into the beam so that the product of the light's spectraloutput, the silicon detector spectral sensitivity, and the filtertransmittance would closely approximate the spectral sensitivity of thehuman eye.

This apparatus was interfaced to a PDP-11/04 computer (marketed byDigital Equipment Corporation, Maynard, Massachusetts) to enableautomatic sample change, temperature selection, event sequencing, anddata collection, storage, reduction, and retrieval with a minimum ofoperator's involvement.

The exposure of three commercially-available photochromic glass samplesto the solar simulator gave the following average values recited below.Approximate analyses in weight percent for each glass are also reported.The glasses marketed under the names PHOTOGRAY® and PHOTOVITAR weremeasured in 2 mm thickness and exhibited clear luminous transmittance ofabout 90-92%, whereas the glass marketed under the name SUNSITIVE™ is asunglass product produced from 1.5 mm thick sheet. That glassdemonstrated a clear luminous transmittance of about 70-72%. Y_(D)designates the darkened transmittance and Y_(F5) represents thetransmittance of the sample five minutes after removal from exposure tothe solar simulator source.

    __________________________________________________________________________    PHOTOGRAY®       PHOTOVITAR             SUNSITIVE™                     __________________________________________________________________________    SiO.sub.2                                                                            55.6          54.0                   58.2                              B.sub.2 O.sub.3                                                                      16.4          16.5                   17.5                              Al.sub.2 O.sub.3                                                                     8.9           8.9                    11.5                              Li.sub.2 O                                                                           2.65          2.37                   2.0                               Na.sub.2 O                                                                           1.85          1.88                   6.7                               K.sub.2 O                                                                            0.01          --                     1.5                               BaO    6.7           9.7                    --                                CaO    0.2           --                     --                                PbO    5.0           0.6                    2.2                               ZrO.sub.2                                                                            2.2           1.9                    --                                Ag     0.16          0.14                   0.18                              CuO    0.035         0.015                  0.018                             Cl     0.24          0.59                   0.24                              Br     0.145         0.18                   0.10                              F      0.19          0.19                   0.23                              MgO    --            2.42                   --                                __________________________________________________________________________    Exposure                                                                              Exposure     Exposure                                                                              Exposure       Exposure                                                                              Exposure                  Temperature                                                                           Time Y.sub.D                                                                           Y.sub.F5                                                                          Temperature                                                                           Time Y.sub.D                                                                            Y.sub.F5                                                                           Temperature                                                                           Time Y.sub.D                                                                          Y.sub.F5          __________________________________________________________________________    40° C.                                                                         20 min.                                                                            58.5%                                                                             76.5%                                                                             40° C.                                                                         20 min.                                                                            62%  86%  40° C.                                                                         20 min.                                                                            32%                                                                              51%               20° C.                                                                         20 min.                                                                            47% 61% 20° C.                                                                         30 min.                                                                            41%  70%  20° C.                                                                         30 min.                                                                            21%                                                                              31%                0° C.                                                                         20 min.                                                                            37.5%                                                                             47.5%                                                                              0° C.                                                                         30 min.                                                                              23.5%                                                                              46.5%                                                                             0° C.                                                                         30 min.                                                                            15%                                                                              19%               -18° C.                                                                        20 min.                                                                            31.5%                                                                             37.5%                                                                             -18° C.                                                                        60 min.                                                                            13%  22%  -18° C.                                                                        60 min.                                                                            14%                                                                              16%               __________________________________________________________________________

Several general conclusions can be reached from a study of the abovedata. First, each glass darkens to a lower transmittance when thetemperature of exposure is lower. The PHOTOVITAR glass does not darkento a very great extent at high ambient temperatures, but darkens to lowlevels at low temperatures. The PHOTOVITAR glass displays more rapidfading than either of the other two specimens, but none of the glassesfades rapidly at low temperatures. This sluggishness in fade rate at lowtemperatures, however, may not be of substantial practical significancesince, in many instances, the glass will be warming up during the fadingprocess. For example, the wearer of ophthalmic lenses will be comingindoors from being outside on a cold day and, as can be observed fromthe above data, the fade rate increases as the temperature rises.

                                      TABLE I                                     __________________________________________________________________________    1        2    3    4    5    6    7    8    9    10   11                      __________________________________________________________________________    SiO.sub.2                                                                         60.4 60.4 60.4 60.4 60.4 60.4 60.4 60.3 59.7 60.4 60.4                    B.sub.2 O.sub.3                                                                   17.7 17.7 17.7 17.7 17.7 17.7 17.7 17.7 17.9 17.7 17.7                    Al.sub.2 O.sub.3                                                                  11.8 11.8 11.8 11.8 11.8 11.8 11.8 11.8 11.5 11.8 11.8                    Li.sub.2 O                                                                        2.1  2.1  2.1  2.1  2.1  2.1  2.1  2.1  2.1  2.1  2.1                     Na.sub.2 O                                                                        5.9  5.9  5.9  5.9  5.9  5.9  5.9  5.9  5.9  6.3  5.9                     K.sub.2 O                                                                         1.6  1.6  1.6  1.6  1.6  1.6  1.6  1.6  1.6  1.6  1.6                     PbO 0.25 0.25 0.25 0.25 0.28 0.28 0.28 0.40 1.0  0.25 0.25                    NiO 0.040                                                                              0.077                                                                              0.120                                                                              0.077                                                                              0.077                                                                              0.185                                                                              --   0.077                                                                              --   0.077                                                                              --                      Co.sub.3 O.sub.4                                                                  0.015                                                                              0.029                                                                              0.046                                                                              0.029                                                                              0.029                                                                              0.013                                                                              --   0.029                                                                              --   0.029                                                                              --                      Ag  0.24/0.17                                                                          0.24/0.17                                                                          0.24/0.17                                                                          0.24/0.17                                                                          0.22/0.15                                                                          0.22/0.15                                                                          0.22/0.16                                                                          0.25/0.15                                                                          0.24/0.16                                                                          0.24/0.14                                                                          0.24/0.14               Cl  0.66/0.47                                                                          0.66/0.47                                                                          0.66/0.47                                                                          0.66/0.48                                                                          0.73/0.47                                                                          0.73/0.49                                                                          0.73/0.48                                                                          0.67/0.38                                                                          0.36/0.27                                                                          0.66/0.40                                                                          0.66/0.39               Br  0.20/0.10                                                                          0.20/0.10                                                                          0.20/0.10                                                                          0.20/0.09                                                                          0.26/0.09                                                                          0.26/0.10                                                                          0.26/0.10                                                                          0.19/0.10                                                                          0.21/0.12                                                                          0.21/0.11                                                                          0.21/0.09               CuO 0.006                                                                              0.006                                                                              0.006                                                                              0.006                                                                              0.0065                                                                             0.0065                                                                             0.0065                                                                             0.006                                                                              0.010                                                                              0.006                                                                              0.006                   F   0.22 0.22 0.22 --   0.22 0.22 0.22 0.23 0.23 0.23 0.23                    __________________________________________________________________________    Measurements at 27° C.                                                 Y.sub.o                                                                           80%  70%  60%  70%  70%  70%  89%  70%  88%  70%  88%                     Y.sub.D10                                                                         29   26   22   27   27   27   30   26   28   26   29                      Y.sub.F5                                                                          63   56   48   56   55   55   64   54   59   55   61                      __________________________________________________________________________    Measurements at 40° C.                                                 Y.sub.o                                                                           80%  70%  60%  70%  70%  70%  89%  70%  88%  70%  88%                     Y.sub.D20                                                                         47   41   35   46   35   35   50   43   43   43   45                      __________________________________________________________________________    Measurements at -18° C.                                                Y.sub.o                                                                           80%  70%  60%  70%  70%  70%  89%  70%  88%  70%  88%                     Y.sub.D20                                                                         25   21   18   26   23   23   29   21   23   21   30                      __________________________________________________________________________

Table II illustrates the completeness of fade or the long term fadingcharacteristics of Examples 5 and 7 of Table I when measured at 27° C.in 1.5 mm thickness. Y_(o) represents the clear luminous transmittance,Y_(D20) and Y_(D60) designate the darkened transmittance after exposuresof 20 minutes and 60 minutes, respectively, to the solar simulatorsource, Y_(F5), Y_(F60), and Y_(F) overnight, indicate the transmittanceafter five minutes, 60 minutes, and an overnight (˜8 hours),respectively, removal from the solar simulator source, and Y_(F60)/Y_(o) reflects the percentage to which the glass has faded after 60minutes with respect to the original luminous transmittance.

                  TABLE II                                                        ______________________________________                                        Ex-                                                                           ample Y.sub.o                                                                              Y.sub.D20                                                                            Y.sub.D60                                                                          Y.sub.F5                                                                           Y.sub.F60                                                                          Y.sub.F overnight                                                                      Y.sub.F60 /Y.sub.o                ______________________________________                                        5     69%    22%    20%  45%  65%  67.3%    94%                               7     89%    28%    26%  60%  83%  86.5%    93%                               ______________________________________                                    

The best possible fade rates can be achieved in those compositions wherePbO is present but at low values. Thus, glasses displaying the mostrapid fading rates, i.e., glasses wherein the luminous transmittanceafter five minutes of fading at 25°-30° C. can exceed 2.25 times thetransmittance in the darkened state, have compositions consistingessentially, in weight percent on the oxide basis as calculated from thebatch, of about 57.1-65.3% SiO₂, 9.6-13.9% Al₂ O₃, 12.0-22.0% B₂ O₃,1.0-3.5% Li₂ O, 3.7-12.0% Na₂ O, 0-5.8% K₂ O, 6-15% total of Li₂ O+Na₂O+K₂ O, the molar ratio Li₂ O:Na₂ O+K₂ O not exceeding about 2:3,0.15-0.7% PbO, 0.10-0.30% Ag, 0.2-1.0% Cl, 0-0.30% Br, 0.002-0.02% CuO,0-2.5% F, 0-1.0% total of transition metal oxides selected in theindicated proportions from the group consisting of 0-0.5% CoO, 0-1.0%NiO, and 0-1.0% Cr₂ O₃, and 0-5.0% total of rare earth metal oxidesselected from the group consisting of Er₂ O₃, Pr₂ O₃, Ho₂ O₃, and Nd₂O₃.

Glasses manifesting similar excellent fading rates which can be drawn assheet and the sheet than simultaneously sagged to yield lenses ofdesired curvatures and photochromic properties developed therein, as hasbeen described above, have compositions falling within the same rangesset out immediately above except for the Ag and Br contents. Thoseconstituents, as analyzed in the glass, will vary as 0.12-0.18% Ag and0.06-0.13% Br.

The photochromic properties of the drawn sheet are self-evidentlyaffected to some degree by the heat treatment employed to develop thoseproperties. This situation is particularly true when the temperatureranges of treatment is strictly limited because of the requirements ofthe simultaneous heat treating-sagging process. However, thoseproperties are also critically dependent upon the composition of theglass. Thus, changes in essentially any of the glass components willresult in modifications of the photochromic behavior. For example, notonly will variations in the "photochromic elements", i.e., silver, thehalides, and copper oxide, alter the photochromic characteristics of aglass, but also, albeit to a lesser extent, will changes in the levelsof alkali metal oxide, SiO₂, B₂ O₃, and PbO.

Table III lists several exemplary glass compositions in parts by weightwithin the scope of U.S. Pat. No. 4,018,965, but outside the scope ofthe instant invention, which demonstrate poor fading characteristics.This failure is attributed to compositional differences. In addition,sheet of Example B cannot be simultaneously heat treated to developdesired photochromic properties while being sagged into lenses havingcurvatures of 4-6 diopters. The concentrations of the glass ingredientsare delineated in parts by weight on the oxide basis as calculated fromthe batch for each glass, except for silver and the halides which arerecorded on the elemental basis. Batch amounts of Ag, Cl, and Br arerecited to the left of the slash marks and analyzed values to the right.The glasses can be compounded and melted in like manner to thedescription underlying Table I. The working examples 1-6 reported inTable I of U.S. Pat. No. 4,018,965 can serve as additional glasscompositions outside the scope of the instant inventive glasses, againdisplaying poor fading characteristics.

                  TABLE III                                                       ______________________________________                                                 A        B          C                                                ______________________________________                                        SiO.sub.2  59.1       59.1       58.2                                         B.sub.2 O.sub.3                                                                          17.5       17.5       17.5                                         Al.sub.2 O.sub.3                                                                         11.5       11.5       11.5                                         Li.sub.2 O 2.0        2.0        2.0                                          Na.sub.2 O 7.7        7.7        6.7                                          K.sub.2 O  --         --         1.5                                          PbO        2.2        2.2        2.2                                          Ag         0.23/0.18  0.27/0.22  0.23/0.18                                    Cl         0.37/0.35  0.37/0.35  0.26/0.24                                    Br         0.15/0.12  0.22/0.19  0.14/0.10                                    CuO        0.023      0.023      0.018                                        F          0.23       0.23       0.23                                         ______________________________________                                    

Table IV reports further exemplary glass compositions in parts byweight, the glasses being within the broad purview of the instantinvention but outside of the preferred ranges of compositions. That is,the glasses demonstrate the desired photochromic properties, but cannotutilize the same heat treatment to develop photochromic behavior whilesagging sheet to desired lens curvatures. Again, batch quantities of Ag,Cl, and Br are recited to the left of the slash mark and analyzed valuesto the right.

                  TABLE IV                                                        ______________________________________                                        12         13       14      15     16    17                                   ______________________________________                                        SiO.sub.2                                                                            60.4    60.4     60.4  60.4   62.0  59.3                               B.sub.2 O.sub.3                                                                      17.7    17.7     17.7  17.7   16.7  17.8                               Al.sub.2 O.sub.3                                                                     11.8    11.8     11.8  11.8   9.4   11.4                               Li.sub.2 O                                                                           2.1     2.1      2.1   2.1    1.9   2.1                                Na.sub.2 O                                                                           5.9     5.9      5.9   5.9    3.8   5.8                                K.sub.2 O                                                                            1.6     1.6      1.6   1.6    4.9   1.6                                PbO    0.25    0.25     0.25  0.25   0.5   1.0                                Ag     0.11/   0.31/    0.31/ 0.25/  0.30/ 0.25/                                     0.08    0.20     0.20  0.22   0.20  0.21                               Cl     0.66/   0.67/    0.37/ 0.35/  0.30/ 0.35/                                     0.39    0.39     0.22  0.31   0.19  0.31                               Br     0.20/   0.20/    0.19/ 0.15/  0.20/ 0.20/                                     0.11    0.09     0.11  0.11   0.12  0.12                               CuO    0.006   0.006    0.006 0.005  0.012 0.010                              F      0.23    0.23     0.23  0.22   --    0.22                               ______________________________________                                    

It is believed that the amount of Ag is too low in Example 12 and toohigh in Examples 13-17.

The optional addition of the above-described transition metal oxide andrare earth metal oxide colorants to the glass compositions of theinstant invention can be useful in securing some light attenuation andcoloration in the faded state, customarily for cosmetic purposes, andalso to provide some coloration and attenuation in the darkened state.Nevertheless, caution must be exercised in selecting colorants for thesephotochromic glasses because the effectiveness of multivalent colorantions is frequently strongly dependent upon the oxidation state of theglass. Furthermore, some colorants absorb ultraviolet radiation, therebyreducing the darkening potential of the glass. For these reasons theforegoing recited transition metal and rare earth metal colorants arepreferred. Nonetheless, minor amounts of additional colloidal or ioniccolorants such as uranium, cadmium sulfide, cadmium selenide, metallicgold, or the like can be included provided such additions do notdeleteriously affect the photochromic properties of the glass.

Table V records specific examples of tinted glass compositions fallingwithin the scope of the instant invention illustrating the use ofseveral of the preferred colorants and the colors induced thereby. Thebase composition for each example was Example 8 of Table I such thatonly the concentrations of the colorants, in parts by weight, aretabulated. The corresponding melting practices utilized with the glassesof Table I were also employed here.

                                      TABLE V                                     __________________________________________________________________________    18       19  20 21  22  23   24  25  26  27                                   __________________________________________________________________________    CoO 0.03 0.03                                                                              0.04                                                                             0.02                                                                              0.01                                                                              --   --  --  --  --                                   NiO 0.08 0.04                                                                              0.03                                                                             0.11                                                                              0.15                                                                              0.17 --  --  --  --                                   Cr.sub.2 O.sub.3                                                                  --   --  -- --  --  --   0.01                                                                              --  --  --                                   Er.sub.2 O.sub.3                                                                  --   --  -- --  --  --   --  0.5 --  --                                   Pr.sub.2 O.sub.3                                                                  --   --  -- --  --  --   --  --  0.5 --                                   Nd.sub.2 O.sub.3                                                                  --   --  -- --  --  --   --  --  --  0.5                                      Neutral                                                                            Blue-                                                                             Blue                                                                             Green-                                                                            Brown                                                                             Yellow-                                                                            Green                                                                             Light-                                                                            Light                                                                             Light                                    Gray Gray   Brown   Brown    Pink                                                                              Green                                                                             Blue                                 __________________________________________________________________________

To aid in further understanding the production practice for fabricatingdrawn sheet glass articles in accordance with the instant invention, thefollowing working example is provided.

EXAMPLE

A glass batch was compounded and melted at a temperature of about 1400°C., the batch having a composition, in parts by weight, of about 60.4SiO₂, 17.7 B₂ O₃, 11.8 Al₂ O₃, 5.9 Na₂ O, 1.6 K₂ O, 2.1 Li₂ O, 0.28 PbO,0.25 Ag, 0.66 Cl, 0.20 Br, 0.23 F, and 0.005 CuO. The molten glass wasfed into a refractory overflow downdraw fusion pipe at a viscosity ofabout 10⁴ poises and delivered from the pipe as drawn sheet about 1.5 mmin thickness. The drawn sheet was cooled below the glass softening pointand separated into sections of sheet glass from which small samples ofdesired geometries were cut. (Analyzed Ag=0.16%, Br=1.10%.)

The sheet glass samples were then exposed to a heat treatment to developphotochromic properties therein, the heat treatment comprising heatingthe samples in a lehr, in a manner such as is described in U.S.application Ser. No. 773,958, supra, that is, edge supported onalveolated molds to prevent surface damage thereto, at a rate of about600° C./hour to 640° C. holding that temperature for 10 minutes to sagthe glass into the concave portions of the alveolated molds, cooling thesamples at 600° C./hour to at least below 450° C., and then removing thesamples from the lehr.

The photochromic glass samples were then subjected to a chemicalstrengthening treatment which involved immersing the samples into a bathof molten NaNO₃ operating at 410° C. for 16 hours. The samples werethereafter removed from the bath, cooled, the excess salt washed offwith tap water, and tested for strength and photochromic properties.

Modulus of rupture values in excess of 45,000 psi were determined andthe depth of the surface compression layers was observed to vary betweenabout 0.0035-0.004".

The fully faded luminous transmittance of a typical 1.5 mm thickphotochromic drawn sheet glass article produced in the manner describedabove is about 90%. After exposure for 60 minutes to the solar simulatorsource at 25° C., a darkened luminous transmittance of about 26% ismeasured. After a five-minute withdrawal from the solar simulatorsource, the glass commonly fades about 34 luminous percentage units to atransmittance of about 60%. The glass will fade to a luminoustransmittance of about 83% after one hour, this value being about 92% ofthe original transmittance.

Upon exposure to the solar simulator source for 60 minutes at 40° C., adarkened luminous transmittance of about 45% is read. At -18° C., adarkened luminous transmittance of about 22% is measured after a60-minute exposure.

The foregoing example, which is merely illustrative and not limitativeof the various compositions and procedures operable in the instantinvention, clearly demonstrates the effectiveness of the inventivecompositions in producing strengthened photochromic drawn sheet glassarticles exhibiting the necessary properties for ophthalmic and otherapplications.

We claim:
 1. A method for forming glass sheet from potentiallyphotochromic glass and thereafter simultaneously shaping articles fromsaid glass sheet and developing photochromic properties therein whichcomprises the steps of:(a) melting a batch for a glass consistingessentially, in weight percent on the oxide basis, of about 57.1-65.3%SiO₂, 9.6-13.9% Al₂ O₃, 12-22% B₂ O₃, 1-3.5% Li₂ O, 3.7-12% Na₂ O,0-5.8% K₂ O, 6-15% total of Li₂ O+Na₂ O+K₂ O, the molar ratio Li₂ O:Na₂O +K₂ +K₂ O not exceeding 2:3, 0.15-0.7% PbO, 0.12-0.18% Ag, 0.2-1% Cl,0.06-0.13% Br, 0-2.5% F, and 0.002-0.02% CuO, the Ag, Cl, and Br rangesrepresenting values as analyzed in the glass; (b) adjusting thetemperature of at least one region of the glass melt to provide aviscosity therein of about 10⁴ -10⁶ poises; (c) drawing the glass meltat a viscosity of about 10⁴ -10⁶ poises directly past platinum orplatinum-clad refractory forming means to produce potentiallyphotochromic drawn glass sheet of optical quality, said glass exhibitinggood resistance to surface crystal growth in contact with platinum suchthat over a contact period of 30 days the growth of a crystalline layerwill not exceed 10 microns in thickness; (d) cooling the glass sheetbelow the softening point of the glass and cutting articles of desiredgeometries therefrom; (e) edge supporting said articles on alveolatedmolds; and then (f) heating said articles at a temperature between about610°-660° C. for a period of time sufficient to simultaneously sag theglass to a desired curvature into the concave portions of the alveolatedmolds, but not into contact with the inner surface of said molds, anddevelop photochromic properties in the glass.
 2. A method according toclaim 1 wherein said time sufficient to simultaneously sag the glass anddevelop photochromic properties therein ranges from about 6-15 minutesat temperatures between 610°-640° C. or from about 5-12 minutes attemperatures between 640°-660° C.
 3. A method according to claim 1wherein said batch also contains up to 1% total of transition metaloxides and/or up to 5% total of rare earth metal oxides as colorants. 4.A method according to claim 3 wherein said transition metal oxides areselected in the indicated proportions from the group consisting of0-0.5% CoO, 0-1.0% NiO, and 0-1.0% Cr₂ O₃, and said rare earth metaloxides are selected from the group consisting of Er₂ O₃, Ho₂ O₃, Nd₂ O₃,and Pr₂ O₃.